It is known to use different pressing machines for increasing the dry matter content of bark produced by debarking of trees. The bark is used as fuel, but before that it has to be dehydrated. The dehydration has been effected by different piston and roller presses. Different press constructions have been described such as in Finnish patent publication 78020. A problem with the known roller press solutions is the feeding of the bark to the narrow pressing nip. The actual pressing time of the roller presses is rather short and for this reason the material must be led through the pressing nip several times. As the bark package loosens between the pressing regions, a part of the pressed water returns. One problem has also been the fact that pressing surfaces perforated for the dehydration must endure very big pressing forces. The dehydration has only been possible to be arranged through the pressing surface below the bark.
The method in accordance with the invention is characterized in that at least one of the pressing surfaces is formed by parallel transfer elements, like balks, moving back and forth in transfer direction, a larger part of the total pressing surface of which is moving in the feeding direction at a time than in the return direction, and that the speed of the return motion of the transfer elements is higher than that of the feed motion. The pressing machine in accordance with the invention is characterized in that at least one of the pressing surfaces is formed by parallel transfer elements, like balks, equipped with devices in order to move them back and forth in feeding direction so, that a larger part of their total pressing surface is moving in the feeding direction at a time than in the return direction, and that the speed of the return motion of the transfer elements is higher than that of the feed motion.
The construction of the pressing machine in accordance with the invention is very simple and solid. The pressing time in the pressing machine can be arranged to be long enough. When the material is going through the press, it is constantly pressed tighter and tighter and it is not necessary to let the material package loosen meanwhile. Feeding of the material can be easily effected. The pressing surfaces can be arranged in a way that the material is fed between them from above, whereby it is possible to effect the dehydration through both pressing surfaces. It is not necessary to perforate the transfer elements on the pressing surfaces, because the dehydration can be arranged through the gaps between the transfer elements. Thus, the transfer elements can be made of acid-proof stainless steel, which is not easily usable in perforated surfaces, as it is very difficult to be perforated. The size of the pressing machine can easily be increased by building new transfer elements next to the earlier ones. If every transfer element is equipped with a separate pressing cylinder, the construction is flexible and yields without being broken by f.ex. stones in bark material.